1. Technical Field
The present invention relates to spark plugs for internal combustion engines.
2. Description of the Related Art
As ignition means in internal combustion engines, there are used spark plugs which include a tubular housing, a tubular insulator, a center electrode and a ground electrode. The insulator is retained in the housing. The center electrode is secured in the insulator with a distal end portion of the center electrode protruding outside the insulator. The ground electrode is provided at a distal end of the housing. The ground electrode has a distal end portion that faces the distal end portion of the center electrode in an axial direction of the spark plug through a spark gap formed therebetween. Those spark plugs are configured to discharge a spark across the spark gap, thereby igniting an air-fuel mixture in a combustion chamber of the engine.
For example, Japanese Patent Application Publication No. JP2009176525A discloses a spark plug which has an annular packing interposed between the insulator and the housing (or main metal body). Specifically, the insulator has a surface that is formed on the outer periphery of the insulator so as to face distalward (i.e., toward the distal side). On the other hand, the housing has a surface that is formed on the inner periphery of the housing so as to face proximalward (i.e., toward the proximal side). The annular packing is interposed between the two surfaces of the insulator and the housing. Further, in the spark plug, the positional relationship between a reduced-diameter portion of the housing and the packing is specified, so as to achieve the cleaning effect due to a corona discharge, thereby improving the anti-fouling capability of the spark plug.
In recent years, it has been aimed to further lower the fuel consumption and improve the efficiency of internal combustion engines. As a consequence, in internal combustion engines, the air-fuel mixture is in an environment where it is difficult for the air-fuel mixture to be combusted; and the combustion temperature is lowered. Further, with the lowering of the combustion temperature, it becomes easier for carbon to be produced, in particular by the combustion of the air-fuel mixture during a cold start of the engine, and adhere to the insulator of a spark plug used in the engine.
In the spark plug disclosed in the above patent document, only the positional relationship between the reduced-diameter portion of the housing and the packing is specified. That is, no improvement is made to the structure of the housing or the packing.
Moreover, in the above patent document, it is aimed to achieve the effect of cleaning (i.e., burning off) the carbon adhered to the insulator by causing a corona discharge to occur while suppressing generation of leak current.
However, the magnitude of corona current which flows during a corona discharge is lower than that of leak current. Consequently, in the spark plug disclosed in the above patent document, it may be impossible to sufficiently burn off the carbon adhered to the insulator.